Personally, I think Mega Diancie and Mega Sableye both do Absol's job better. Both get Magic Bounce, but have a lot more bulk to work with, whereas Absol can be OHKO'd by most neutral hits.
I am not completely up-to-date with the current tiers, but I know Absol is UU. I don't know about Sableye or DIancie, although I think their regular forms are UU. If their Megas are banned from UU, then Absol might be worth the Mega slot in UU. If not, then it is simply outclassed.